gitlab: add CI job for validating DCO signoff
gitlab: add CI job for validating DCO signoff

This job uses the shared "check-dco" image to validate that all
commits on a branch in a developer's repo fork have a suitable
Signed-off-by statement present.

Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

zippy2/libvirt-php,libvirt/libvirt-php,zippy2/libvirt-php,libvirt/libvirt-php,zippy2/libvirt-php,libvirt/libvirt-php